Add fsync/full_page_writes optimization during massive sync

Disable WAL safety features (fsync, full_page_writes) during massive sync
for better performance. Original values are saved and restored after
massive sync completes, preserving any custom configuration (e.g., ZFS
setups with full_page_writes=off).

This is safe because if massive sync is interrupted, the database is in
an inconsistent state anyway and must be rebuilt from scratch.